Methanol is a simple, safe liquid oxygenous hydrocarbon fuel and can be easily stored and transported. Methanol is also an important chemical raw material which can be used to replace petroleum to synthesize a variety of products, such as methanol to olefins (MTO), methanol to propylene (MTP), methanol to aromatics (MTA) and so on. As an important energy carrier, methanol is an effective solution dealing with energy issues resulting from the depletion of petroleum, coal, natural gas in post oil-gas era [Olah, G. A., Geoppert, A. & Surya Prakash, G. K. Beyond Oil and Gas. The Methanol Economy, Wiley-VCH, 2006.]. In recent years, methanol production scale has increased rapidly (merely in China, 38 million tons of methanol was produced in 2010), however, such processes use coal, natural gas or petroleum as raw material, and, therefore, will face the challenge of depletion of fossil fuel resources, and such resources are difficult to regenerate.
On the other hand, diol compounds are fuel and industrial raw materials of significant uses. For example, 1,2-ethylene glycol is an extremely important chemical raw material and solvent, which can be widely used in polyester fibers, films, resins and engine coolants. Traditionally, 1,2-ethylene glycol is prepared by hydrolysis of ethylene oxide, however, a variety of produced byproducts directly affect the quality of the product. To address this issue, a new process called OMEGA was developed by Shell (UK), in which cyclic ethylene carbonate is firstly obtained from ethylene oxide and carbon dioxide, and 1,2-ethylene glycol is obtained by hydrolysis of the cyclic ethylene carbonate under a catalyst with emission of carbon dioxide [http://en.wikipedia.org/wiki/OMEGA_process#cite_note-0.]. 1,2-ethylene glycol can be obtained by OMEGA process at 99% selectivity, however, the deficiency for this process is that carbon dioxide can not be effectively utilized and released back to the environment upon reaction.
Carbon dioxide is a greenhouse gas affecting environment, however, carbon dioxide is also a inexhaustible, inexpensive, safe and renewable carbon resources [Carbon Dioxide as Chemical Feedstock, (Ed.: M. Aresta), Wiley-VCH, Weinheim, 2010)]. Catalytic hydrogenation of carbon dioxide to produce methanol is an important passway to realize methanol economy. This reaction is thermodynamically feasible, however, carbon dioxide is generally very difficult to be directly reduced, which is the main challenge in the industry and academia, due to the high inertia of carbon-oxygen double bond. At present, a few heterogeneous catalyst systems have been reported for this reaction, however, a prevalent defect in it is that the reaction should be performed under strict conditions, such as high temperature and high pressure (250° C., 50 atm), and the efficiency and selectivity of the reaction are not high enough [W. Wang, S. P. Wang, X. B. Ma, J. L. Gong, Chem. Soc. Rev. 2011, 40, 3703-3727]. On the other hand, currently only two cases were reported, which refer to the direct hydrogenation of carbon dioxide into methanol by using a homogeneous catalyst system and hydrogen as a hydrogen source, however, the catalytic efficiency of the two methods is relatively low (the highest conversion number for catalyst is ≦221) [C. A. Huff, M. S. Sanford, J. Am. Chem. Soc. 2011, 133, 18122-18125], [S. Wesselbaum, T. vom Stein, J. Klankermayer, W. Leitner, Angew. Chem., Int. Ed. 2012, 51, 7499-7502]. So far, there is no practical catalytic process for catalytically hydrogenating carbon dioxide into methanol. On the other hand, in 2011, Milstein et al., reported a method for hydrogenating dimethyl carbonate to synthesize methanol by using ruthenium complex as homogeneous catalyst [E. Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon, D. Milstein, Nature Chem. 2011, 3, 609-614]. In the prior art, dimethyl carbonate is obtained through transesterification between cyclic ethylene carbonate and methanol (Aresta, M. (ed.) Carbon Dioxide as Chemical Feedstock (Wiley-VCH, 2010). However, the market price of dimethyl carbonate is significantly higher than that of methanol, so that such catalytic system obviously has no practicability and economic value. Moreover, it is well-known to a person skilled in the art that the structure of used ligand will greatly affect catalytic activity of the resulting complex, when using a metal complex as catalyst; in other words, small changes in the structure of ligand will result in the loss of catalytic activity of the resulting metal complex.
Summing up, there is an urgent need in the art for a method to directly or indirectly convert carbon dioxide into methanol under mild conditions, and such method not only has a high conversion efficiency, but also has excellent economy. Additionally, in the prior art, degradation of polycarbonate materials relys on hydrolysis, the degradation products are diol and carbon dioxide, wherein carbon dioxide will release back into environment again. Therefore, there is an urgent need in the art for a method to degrade polycarbonate through hydrogenation and reduction under mild conditions.